The iron pillar of Delhi, India, is a 7 m (23 ft) high pillar in the Qutb complex, notable for the composition of the metals used in its construction.

The pillar, which weighs more than six tons, is said to have been fashioned at the time of Chandragupta Vikramaditya (375–413), though other authorities give dates as early as 912 BCE. The pillar initially stood in the centre of a Jain temple complex housing twenty-seven temples that were destroyed by Qutb-ud-din Aybak, and their material was used in building the Qutub Minar and Quwwat-ul-Islam mosque. The pillar and ruins of the temple stand all around the Qutb complex today. The pillar is 98% pure wrought iron and is a testament to the skill of ancient Indian blacksmiths. It has attracted the attention of both archaeologists and metallurgists, as it has withstood corrosion for over 1,600 years in the open air.

The name of the city of Delhi is thought to be based on a legend associated with the pillar (see History of Delhi).

Description

The height of the pillar, from the top of its capital to the bottom of its base, is 23 ft 8 in (7.21 m), 3 ft 8 in (1.12 m) of which is below ground. Its bell pattern capital is 3 ft 6 in (1.07 m) in height, and its bulb-shaped base is 2 ft 4 in (0.71 m) high. The base rests on a grid of iron bars soldered with lead into the upper layer of the dressed stone pavement. The pillar's lower diameter is 16.4 in (420 mm), and its upper diameter 12.05 in (306 mm). The bell pattern capital is 3 ft 6 in (1.07 m) high. It is estimated to weigh more than six tons.

The pillar was erected by Chandragupta Vikramaditya (375 CE–414 CE), (interpretation based on analysis of archer-type Gupta gold coins) of the Gupta dynasty that ruled northern India 320–540. The pillar, with the statue of Chakra at the top, was originally located at a place called Vishnupadagiri (meaning "hill with footprint of Lord Vishnu"). This has been identified as modern Udayagiri, situated in the vicinity of Vidisha, Madhya Pradesh. There are several aspects to the original site of the pillar at Udayagiri. Vishnupadagiri is located on the Tropic of Cancer and, therefore, was a centre of astronomical studies during the Gupta period. The iron pillar served as a sundial when it was originally at Vishnupadagiri. The early-morning shadow of the iron pillar fell in the direction of the foot of Anantasayin Vishnu (in one of the panels at Udayagiri) only around the summer solstice (June 21). The Udayagiri site in general, and the iron pillar location in particular, are evidence for the astronomical knowledge that existed in Gupta India.

The pillar bears a Sanskrit inscription in Brahmi script, which states that it was erected as a standard in honour of Lord Vishnu. It also praises the valor and qualities of a king referred to simply as Chandra, who has been identified with the Gupta King Chandragupta Vikramaditya (375-413). The inscription reads (in the translation given in the tablets erected by Pandit Banke Rai in 1903):

It is believed by some that the pillar was installed in its current location by Vigraha Raja, the ruling Tomar king. One of the inscriptions on the iron pillar from A.D. 1052 mentions Tomara king Anangpal II.

Made up of 98% pure wrought iron, it is 7.21 m (23 feet 8 inches) high, with 93 cm (36.6 inches) buried below the present floor level, and has a diameter of 41 cm (16 inches) at the bottom, which tapers towards the upper end. The pillar was manufactured by forge welding. The temperatures required to form such a pillar by forge welding could only have been achieved by the combustion of coal. The pillar is a testament to the high level of skill achieved by ancient Indian blacksmiths in the extraction and processing of iron.

A fence was erected around the pillar in 1997 in response to damage caused by visitors. There is a popular tradition that it was considered good luck if one could stand with one's back to the pillar and make one's hands meet behind it.

Scientific analysis

In a report published in the journal Current Science, R. Balasubramaniam of the IIT Kanpur explains how the pillar's resistance to corrosion is due to a passive protective film at the iron-rust interface. The presence of second-phase particles (slag and unreduced iron oxides) in the microstructure of the iron, that of high amounts of phosphorus in the metal, and the alternate wetting and drying existing under atmospheric conditions are the three main factors in the three-stage formation of that protective passive film.

Lepidocrocite and goethite are the first amorphous iron oxyhydroxides that appear upon oxidation of iron. High corrosion rates are initially observed. Then, an essential chemical reaction intervenes: slag and unreduced iron oxides (second phase particles) in the iron microstructure alter the polarization characteristics and enrich the metal–scale interface with phosphorus, thus indirectly promoting passivation of the iron (cessation of rusting activity). The second-phase particles act as a cathode, and the metal itself serves as anode, for a mini-galvanic corrosion reaction during environment exposure. Part of the initial iron oxyhydroxides is also transformed into magnetite, which somewhat slows down the process of corrosion. The ongoing reduction of lepidocrocite and the diffusion of oxygen and complementary corrosion through the cracks and pores in the rust still contribute to the corrosion mechanism from atmospheric conditions.

The next main agent to intervene in protection from oxidation is phosphorus, enhanced at the metal–scale interface by the same chemical interaction previously described between the slags and the metal. The ancient Indian smiths did not add lime to their furnaces. The use of limestone as in modern blast furnaces yields pig iron that is later converted into steel; in the process, most phosphorus is carried away by the slag. The absence of lime in the slag and the deliberate use of specific quantities of wood with high phosphorus content (for example, Cassia auriculata) during the smelting induces a higher phosphorus content (> 0.1%, average 0.25%) than in modern iron produced in blast furnaces (usually less than 0.05%). One analysis gives 0.10% in the slags for 0.18% in the iron itself. This high phosphorus content and particular repartition are essential catalysts in the formation of a passive protective film of misawite (d-FeOOH), an amorphous iron oxyhydroxide that forms a barrier by adhering next to the interface between metal and rust. Misawite, the initial corrosion-resistance agent, was thus named because of the pioneering studies of Misawa and co-workers on the effects of phosphorus and copper and those of alternating atmospheric conditions in rust formation.

The most critical corrosion-resistance agent is iron hydrogen phosphate hydrate (FePO4-H3PO4-4H2O) under its crystalline form and building up as a thin layer next to the interface between metal and rust. Rust initially contains iron oxide/oxyhydroxides in their amorphous forms. Due to the initial corrosion of metal, there is more phosphorus at the metal–scale interface than in the bulk of the metal. Alternate environmental wetting and drying cycles provide the moisture for phosphoric-acid formation. Over time, the amorphous phosphate is precipitated into its crystalline form (the latter being therefore an indicator of old age, as this precipitation is a rather slow happening). The crystalline phosphate eventually forms a continuous layer next to the metal, which results in an excellent corrosion resistance layer. In 1,600 years, the film has grown just one-twentieth of a millimetre thick.

Balasubramaniam states that the pillar is "a living testimony to the skill of metallurgists of ancient India". An interview with Balasubramaniam and his work can be seen in the 2005 article by Veazy.

It was claimed in the 1920s that iron manufactured in Mirjati near Jamshedpur is similar to the iron of the Delhi pillar. Further work on Adivasi (tribal) iron by the National Metallurgical Laboratory in the 1960s did not verify this claim.

Research published in 2010 showed that corrosion has developed evenly over the surface of the pillar.